A copper-zinc alloy is excellent in workability and has been widely used in various fields in the related art. In general, the material costs of the copper-zinc alloy may be reduced by increasing the zinc content because the zinc base metal is cheaper than the copper base metal. Further, when the zinc content is in a range of 43% by weight or less, cold working with a rolling reduction of 80% or more may be performed and strength may be improved by the processing deformation generated in the cold working, and the higher the zinc content is, the more improved effects are obtained.
In addition, the copper-zinc alloy is known to exhibit an inherent alloy color tone depending on the zinc content thereof. For example, a copper-zinc alloy containing zinc in an amount of 15% by weight (generally referred to as red brass) has a reddish gold color tone. Furthermore, a copper-zinc alloy containing zinc in an amount of 30% by weight (generally referred to as seven-three brass) has a yellowish gold color tone, and a copper-zinc alloy containing zinc in an amount of 40% by weight (generally referred to as four-six brass) has a reddish gold color tone as in the red brass.
As for the copper-zinc alloy, in order to further improve properties such as strength, resistance to corrosion, or the like, various research and development activities have been conducted in the related art and put into practical use.
For example, Japanese Patent Application Laid-Open No. 2000-129376 (Patent Document 1) discloses a copper-zinc alloy with the strength improved without deteriorating the workability.
The copper-zinc alloy disclosed in Patent Document 1 contains copper in an amount of 60% by weight or more and less than 65% by weight. Further, the metal structure of the copper-zinc alloy has a two-phase mixed structure composed of fine α-phase and β-phase, except for the coarse β-phase that inevitably remains and the non-recrystallized α-phase. According to Patent Document 1, the strength is not increased in a copper content of 65% by weight or more and the workability is not sufficient in a copper content of less than 60% by weight.
Further, in Patent Document 1, the two-phase mixed structure composed of the fine α-phase and β-phase is said to mean a state in which the β-phase with a size of from 0.1 μm to 2 μm is present while being in contact with the α-phase at the grain boundary. In addition, the β-phase which is inevitably present is said to be a β-phase which is present before a low-temperature annealing or a coarsely growing β-phase which is partially generated from a processed structure during the low-temperature annealing, and the non-recrystallized α-phase is said to mean that a processed structure partially remains while the processed structure is transformed into a two-phase mixed structure during the low-temperature annealing treatment.
When the copper-zinc alloy in Patent Document 1 is produced, an alloy is obtained by first melting a raw material having a predetermined composition, casting the melt, and subjecting the melt to hot working, and then the alloy obtained is subjected to a cold working with a cold working ratio of 50% or more.
After the cold working with a cold working ratio of 50% or more, the alloy is subjected to a low-temperature annealing. Accordingly, the β-phase is created while simultaneously removing the processing deformation. In this case, according to Patent Document 1, it takes time to create the β-phase when the temperature of the low-temperature annealing is low, and the recrystallized α-phase appears when the temperature of the low-temperature annealing is high, thereby making it impossible to obtain a sufficient strength, and thus it is preferred to set the temperature of the low-temperature annealing at approximately from 200° C. to 270° C. According to Patent Document 1, a copper-zinc alloy produced by performing the low-temperature annealing may improve the strength thereof without degrading the workability such as press bendability and the like.
On one hand, for example, Japanese Patent Application Laid-Open No. 2000-355746 (Patent Document 2) discloses a copper-zinc alloy having a zinc content of from 37% by weight to 46% by weight, an α+β crystal structure at normal temperature, a β-phase area ratio of 20% or more in the crystal structure at normal temperature, and an average crystal particle diameter of the α-phase and the β-phase of 15 μm or less, and describes that this type of copper-zinc alloy has excellent cutting performance and strength.
Further, according to Patent Document 2, the copper-zinc alloy is produced by subjecting a copper-zinc alloy material having a zinc content of from 37% by weight to 46% by weight to hot extrusion at a temperature in a range from 480° C. to 650° C. and then cooling the copper-zinc alloy material at 0.4° C./sec or higher until the temperature is 400° C. or less.